1. Field of the Invention
The present invention relates to a film adopted for use in an intermediate image transfer member, which may be used in the form of an endless belt, to which a toner image can be transferred, and which is capable of reducing the formation of image transfer dust and the deposition of toner on the background of images in an image formation process.
The present invention also relates to an endless-belt-shaped film made of the above-mentioned film for use in the intermediate image transfer member.
2. Discussion of Background
Conventionally, there is known an image formation apparatus in which a latent electrostatic image is formed on a latent image bearing member such as a photoconductor drum, the latent electrostatic image is developed with toner to a visible toner image, the visible toner image is then electrostatically transferred to an endless-belt-shaped intermediate image transfer belt, and the toner image is further transferred from the intermediate image transfer belt to a recording material such as a transfer sheet.
In this kind of image formation apparatus, an electroconductive intermediate image transfer belt is widely used as the above-mentioned intermediate image transfer belt.
However, it is difficult to set the surface resistivity of the electroconductive intermediate image transfer belt at an appropriate value. For example, in Japanese Patent 2560727, it is described that proper images can be formed when the surface resistivity of the electroconductive image transfer image transfer belt is in a range of 107 to 1015 [xcexa9/xe2x96xa1]. However, there is a case where the deposition of toner on the background of images occurs and image transfer dust is formed, with a toner being transferred away from its right image transfer position even when the surface resistivity of the electroconductive image transfer belt is set in the above-mentioned range. It is also known that such image transfer dust occurs frequently when an intermediate image transfer belt with lower surface resistivity is used. Such image transfer dust is considered to be caused by the electric lines of force, formed between the intermediate image transfer belt and the latent electrostatic image bearing member, being disturbed in the direction of the surface of the intermediate image transfer belt. Furthermore, it is known that the deposition of toner on the background of images occurs frequently when an intermediate image transfer belt with higher surface resistivity is used. Thus, there is a dilemma in which when the surface resistivity of the intermediate image transfer belt is increased in order to reduce the formation of the image transfer dust, the deposition of toner on the background of images is increased, while when the surface resistivity of the intermediate image transfer belt is decreased in order to reduce the deposition of toner on the background of images, the occurrence of the image transfer dust is promoted.
In order to get out from such a dilemma, it will be necessary to set the surface resistivity of the intermediate image transfer belt at a high surface resistivity and also to provide a charge quenching unit for quenching electric charges of the intermediate image transfer belt. However, the provision of such a charge quenching unit not only makes the image formation apparatus costly, but also makes the mechanism of the image formation apparatus complex. Therefore, the provision of the quenching apparatus is not a preferable step from the above point of view. Furthermore, a corona charger, which is one of the simplest quenching apparatus, is considered to be usable for the above-mentioned purpose. However, the corona charge has a shortcoming that it generates ozone and causes an air pollution problem.
It is therefore a first object of the present invention to provide a film with a single layer structure, which can be adopted for use in an intermediate image transfer member, and may be used in the form of an endless belt, to which a toner image can be transferred, and which is capable of reducing the formation of image transfer dust and the deposition of toner on the background of images in an image formation process, without the necessity for the provision of any charge quenching apparatus.
A second object of the present invention is to provide a method of producing an endless-belt-shaped film made of the above-mentioned film for use in the intermediate image transfer member.
The first object of the present invention can be achieved by a film which comprises an insulating matrix resin and an electroconductive material, in which the electroconductive material is dispersed in the insulating matrix resin in such a manner that a surface resistivity of A [xcexa9] of the film and a volume resistivity of B [xcexa9xc2x7cm] of the film in the direction of a thickness thereof normal to the surface of the film satisfy a relationship of:
A greater than B. 
The first object of the present invention can also be achieved by a film comprising an insulating matrix resin and an electroconductive material, having a thickness of T [cm], the electroconductive material being dispersed in the insulating matrix resin in such a manner that a surface resistivity of A [xcexa9] of the film and a volume resistivity of B [xcexa9xc2x7cm] of the film in the direction of a thickness thereof normal to the surface of the film satisfy a relationship of:
A[xcexa9]xc3x97T[cm] greater than B[xcexa9xc2x7cm]. 
In any of the above films, the electroconductive material may comprise a first electroconductive material and a second electroconductive material, each of which has a different resistivity.
Furthermore, in any of the above films, the electroconductive material may comprise a first electroconductive material and a second electroconductive material, each of which has a different particle size.
Furthermore, in any of the above films, the electroconductive material may be in the shape of needles.
Each of the needles of the electroconductive material may be in such a shape that the thickness thereof is decreased toward opposite end portions thereof.
As the above-mentioned electroconductive material, an electroconductive material comprising carbon can be used.
The first object of the present invention can also be achieved by a film with a single layer structure comprising a first region extending along a surface of the film and a second region extending under the first region, the first region comprising an insulting matrix resin and a first electroconductive material dispersed in the form of particles in the insulating matrix resin, and the second region comprising the insulating matrix resin and a second electroconductive material dispersed in the form of particles in the insulating matrix resin, the first electroconductive material having a lower electroconductivity than that of the second electroconductive material.
Furthermore, the first object of the present invention can also be achieved by a film with a single layer structure comprising a first region extending along a surface of the film and a second region extending under the first region, the first region comprising an insulting matrix resin and a first electroconductive material dispersed in the insulating matrix resin, and the second region comprising the insulating matrix resin and a second electroconductive material dispersed in the insulating matrix resin, the first electroconductive material and the second electroconductive material being of an identical electroconductive material, and the first electroconductive material having a larger particle size than that of the second electroconductive material.
Furthermore, the first object of the present invention can also be achieved by a film with a single layer structure comprising an insulting matrix resin and an electroconductive material dispersed in the form of needles in the insulating matrix resin, with a longitudinal side of the needles of the electroconductive material being oriented in the direction normal to an external surface of the film.
In the above film, it is preferable that a cross section of each of the needles of the electroconductive material, in the direction normal to the external surface of the film, be in the shape of a quadrilateral with unequal diagonal lines, a longer diagonal line being oriented in the direction normal to the external surface of the film and a shorter diagonal line being oriented in the direction normal to the longer diagonal line.
Furthermore, it is preferable that the above quadrilateral have a pair of equal adjacent sides directed to the external surface of the film, and a pair of equal adjacent sides directed to a back side of the film.
Furthermore, it is also preferable that the pair of equal adjacent sides of the quadrilateral directed to the external surface of the film be shorter than the pair of equal adjacent sides of the quadrilateral directed to the back side of the film.
The second object of the present invention can be achieved by a method of producing an endless-belt-shaped film with a single layer structure comprising an insulting matrix resin and an electroconductive material dispersed in the form of particles in the insulating matrix resin, with larger particles of the electroconductive material being positioned on the side of an external surface of the endless-belt-shaped film, and smaller particles of the electroconductive material being positioned on the side of an inner surface of the endless-belt-shaped film, comprising the steps of:
dispersing the electroconductive materials with different particles size in a solution of the matrix resin in a solvent to form a dispersion of the electroconductive materials in the solution of the matrix resin, and
subjecting the dispersion to centrifugal molding to localize larger particles of the electroconductive material on the side of the external surface of the endless-belt-shaped film, and to localize smaller particles of the electroconductive material on the side of the inner surface of the endless-belt-shaped film, with removal of the solvent therefrom.